power: supply: smb347-charger: Utilize generic regmap caching

Utilize generic regmap caching in order to avoid unnecessary slow I2C
accesses to all constant registers each time the supply status updated
and remove local caching of charger state to make code cleaner.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>